The platelet-derived growth factor (PDGF) family of disulfide-linked dimeric proteins consists of four homodimeric proteins, PDGF-AA, PDGF-BB, PDGF-CC and PDGF-DD, and one heterodimeric protein, PDGF-AB. Members of this protein family act mainly on connective tissue. The dimeric isoforms of PDGF are expressed by different cell types and at different times during embryonic development. All PDGF isoforms are synthesized as inactive precursors and are processed to active forms that contain a PDGF/VEGF cysteine knot domain. Only PDGF-C and PDGF-D, also named spinal cord-derived growth factors SCDGF and SCDGF-B, also contain an N-terminal CUB domain. Mature PDGF dimers bind and induce the homo- or hetero-dimerization of two receptor tyrosine kinases (PDGF R alpha and R beta).